Feedback is very large subject, and I am going to limit myself to some simple tutorial comments and a discussion of phenomena associated with complexity in distortion created by nonlinear gain stages, negative feedback, and the audio signal. Taken singly, these phenomena seem simple enough, but when they interact, they create distortions out of proportion to what you expect from the specifications found in product brochures.

First a little background. When we talk about feedback and nonlinear distortion, we are inevitably talking about gain devices – Tubes, Bipolar transistors, Jfets and Mosfets, and when we talk about gain devices, we usually get around to nonlinear distortion. Here are the basic players shown in simple circuits:

All of these examples have current flowing from pin 1 to pin 2 as a function of the voltage between pin 3 and pin 2. In the example of the tube, the current from Plate to Cathode is largely determined by the voltage between the Grid and the Cathode. The other examples have similar relationships, but vary in the details. We think of pin 3 as the control pin, and signal presented here is amplified to a larger signal passing through the other two pins.